Security or condition sensors such as infrared detectors comprise one of the major components of detection systems. One problem with prior art detection systems, however, is the number of false alarms triggered by spurious stimuli unrelated to a legitimate event in the protected area. Among such stimuli that cause false triggering are cycling on and off of heaters within the field of view of the detector; visible and near infrared energy entering the detector field of view from high intensity light sources such as automobile headlights; mechanical shock and vibration; air drafts; and random internally generated spike noise produced by the detector.
Prior art detection systems have attempted to minimize or eliminate false alarm triggering by employing pulse counting or frequency discrimination methods. Although signals produced by the spurious stimuli have characteristic differences from the signals of legitimate events, simple pulse counting or frequency discrimination methods employed in the prior art have not proven completely effective for reasonably limiting or eliminating false alarm triggering while still retaining adequate detection of legitimate events.
The step response function of a conventional signal processing amplifier also contributes to the problems with prior art systems. Such an amplifier will produce a signal overshoot in response to unwanted stimuli. This signal overshoot causes two counts to be registered by conventional pulse counting circuitry in response to what was in reality only a single event. Accordingly, some systems have resorted to three count logic to avoid false alarm triggering. The use of three count logic, however, makes detection of a legitimate event by a single field of view detector more unlikely.